1. Field of the Invention
This invention relates to the desulfurization of molten ferrous metals; more particularly to the controlled injection of a mixture of non-oxidizing material and carbon-containing particles into molten iron to achieve desulfurization.
2. Description of the Prior Art
The present invention is an improvement of the invention disclosed and claimed in U.S. Pat. No. 3,998,625, which is owned by the assignee of this application and which is incorporated by reference herein.
U.S. Pat. No. 3,998,625 discloses a desulfurization process in which a particulate non-oxidizing material such as lime and particulate magnesium-containing material are separately fed from their respective storage means to form a fluidized mixture in a non-oxidizing carrier gas and this mixture is injected into a molten ferrous metal. The magnesium component of the injected mixture serves as a potent desulfurization agent in the ferrous metal. A principal advantage of the process taught in U.S. Pat. No. 3,998,625 is that the injection rate of magnesium-containing material may be varied during the injection period to take into account process variables such as the fact that the efficiency of magnesium desulfurization decreases as the sulfur content of the bath decreases.
The lime/magnesium process disclosed in U.S. Pat. No. 3,998,625 has proven to be a commercial success. However, the magnesium-containing component used in the process is relatively expensive and this factor prompted further efforts by workers in the art to reach a less expensive but equally effective desulfurization method.
West German Offenlegungsschrift No. 2,301,987 describes a desulfurization process in which fine lime and finely granulated saturated hydrocarbons are mixed and then injected into molten iron, preferably with a carbon monoxide-containing carrier gas. The Offenlegungsschrift teaches that the lime/hydrocarbon mixture should contain about 5% hydrocarbons by weight but that the proportion may go as high as 20% by weight.
In a later filed West German application by the same applicant, Offenlegungsschrift No. 2,337,957, an alleged improvement of the lime/hydrocarbon injection process is described; the improvement consists of coating the fine lime particles with the hydrocarbons. Again it is stated that the coated lime particles should contain hydrocarbons in the range of 5 to 20% by weight.
Neither of the West German Offenlegungsschrifts specifies a particular hydrocarbon for use in the process; the suitable hydrocarbons are described only by reference to the formula C.sub.n H.sub.2n +.sub.2', which identifies a saturated hydrocarbon from the alkane family. Further, neither Offenlegungsschrift mentions any proportions of lime or hydrocarbon in relationship to the quantity of molten iron to be desulfurized; nor is any mention made of injection rates or other process operating parameters.
It has been found that the introduction of solid hydrocarbons mixed with finely divided lime into a molten iron bath tends to produce violent agitation within the bath when the hydrocarbon injection rates are relatively high; if the molten iron is carried in a conventional submarine ladle, this agitation manifests itself as undesirable splashing or slopping when the ladle is filled to design capacity. The cause of this violent agitation is the dissociation of the hydrocarbon when it contacts the molten bath and attendant release of hydrogen gas within the bath.
If, for example, a simple conversion from the lowest recommended weight percent taught by the above-referenced Offenlegungsschrifts, 5% by weight, to a hydrocarbon injection rate can be made, that weight percent represents a hydrocarbon injection rate of about 6.8 lb./min. based on a lime injection rate of 130 lb./min.; this lime injection rate, according to U.S. Pat. No. 3,998,625, is deemed desirable for smooth operation in desulfurizing pig iron having typical sulfur contents. Injecting hydrocarbon, for example polypropylene, at a rate of over 6 lb./min. results in splashing within the ladle that can be tolerated only by a drastic reduction in the quantity of molten metal carried in the ladle. The even higher hydrocarbon injection rates that would result from observing the upper end of the hydrocarbon weight percent range suggested in the German process are clearly inappropriate.
A process using polypropylene mixed with lime would, if the German teachings were observed, involve an additional disadvantage. U.S. Pat. No. 3,998,625 teaches that the lime particles preferably should be sized so that 98% are less than about 44 microns. If the German teaching is followed, specifically the teaching that the solid hydrocarbons and the fine lime should have approximately the same grain size, preferably less than 1 mm, the polypropylene particles should be substantially of the same size. But when the grain size of polypropylene is reduced below about 75 microns, the material is pyrophoric and a dust explosion hazard is presented.